This invention relates to devices and methods for transferring thermal energy to a patient, and more particularly vascular catheters and systems for dispelling hypothermia.
It is common for patients surgical procedures, to experience mild to severe hypothermia. There are numerous causes for this decrease of body temperature. One cause pertains to anesthesia. Anesthesia may depress the body""s set-point temperature as regulated by the brain""s thermal control center.
Another cause for decreased body temperature during surgery may manifest when the patient has his or her thoracic or abdominal cavity opened. This greatly increases the amount of surface area exposed to the atmosphere and thereby accelerates loss of body heat. As a rule, surgical suites are kept very cool. Cool surgical suites can make patients cold.
Several surgical procedures (e.g., coronary bypass grafts, valve replacements, etc.) utilize intentional hypothermia in order to decrease the body""s energy demands during the procedure. These patients need to return from a deep hypothermic state to a normal body temperature following the procedure.
Current methods of treating the hypothermic patient include hot baths, delivering warm fluids orally and applying heating blankets. Heating blankets use either air or liquid as the heat-transfer medium. If placed below the patient, a heating blanket transfers thermal energy to the patient by a combination of conduction and convection. Conductive heating results from intimate, pressured, contact with the skin of a patient. Convective heating results from using a local air film between the blanket surface and a patient""s skin to convectively heat the patient.
Some air type heating blankets are placed over the patient, and supply warm air under low pressure via the blanket. The warm air xe2x80x9cleaksxe2x80x9d out of the blanket at low velocity and with reasonable uniformity over its surface to warm the patient. Heating blankets transfer heat through the patient""s skin surface, relying principally on the vascular system to transfer the thermal energy to the patient""s core via the blood flow.
The rate of heat transfer, and therefore the effectiveness, of a heat-blanket warming system is limited by the body""s natural response to low core temperatures. Such a response may include xe2x80x9cshutting downxe2x80x9d blood flow to the extremities. The body may also sweat in response to application of a heating blanket. Sweating cools the body and reduces the effectiveness of heating blankets.
The heating blanket approach is sometimes used in conjunction with other ways of heating the patient, such as heating the blood supply directly. U.S. Pat. No. 5,837,003 to Ginsburg, issued Nov. 17, 1998, discloses an exemplary way of using an electrically resistive heater at the end of a catheter to heat the blood supply of a patient. Heating the blood supply, in conjunction with blankets and other ways of heating the patient, can shorten the time required to bring a patient to normothermia.
One drawback to the invention disclosed by Ginsburg is that in order to transfer a significant amount of heat to the blood supply, the resistive element would have to be very hot. It is known, however, that blood heated over 42 degrees C., or so, may coagulate. Accordingly, the maximum temperature at which the resistive elements operate is restricted by the tendency of the blood to coagulate.
In an effort to provide a catheter that can transfer heat at relatively lower temperatures, the catheter surface area has been increased. One design increases the distal diameter of the catheter (See U.S. Pat. No. 5,837,003 FIG. 7). Other designs show helical fins, annular fins and axial fins, respectively. (U.S. Pat. No. 5,837,003 FIGS. 8a, 8b, and 8c). The fins maximize the surface area of the catheter in contact with the blood supply and, thereby, improve the ability of the catheter to conduct heat to the blood supply.
Maximizing the area of a heating surface on a catheter is not wholly effective. One reason for this is that increasing the diameter of a catheter impedes blood flow, which, may reduce the effectiveness of any heat transfer between the catheter and the blood supply. Further, fins may impede blood flow in regions adjacent the fins, causing the blood to overheat. Blood that overheats could coagulate on the fins.
Fins, by themselves, are somewhat inefficient. Much of the blood that passes the catheter does not contact the fins or heat exchange elements and therefore, may not undergo a significant temperature change.
What is desired is a catheter that does not significantly impede blood flow. What is also desired is a catheter with improved heat transfer capability without overheating the blood.
The system of the present invention includes a heat exchange catheter for warming flowing blood within a blood vessel, or for warming tissue in any body cavity. The heat exchange catheter includes a catheter body having a proximal end and a distal end with electrodes and temperature sensor elements.
The electrodes generate an electric field that radiates into the flowing blood. Heating of the blood results when the electric field exerts one of two possible effects on the blood, depending on the frequency of the energy and whether the electrodes are in direct ohmic contact with the blood.
It is understood that the electric field accelerates free electrons in the blood, creating a flow of localized electric currents in the blood. The electric currents flowing through the blood cause resistive heating of the fluid through the relationship P=I2R, where P is power, I is the RMS current and R is the blood resistance.
It is also understood that the electric field is absorbed, principally by the water molecules that make up the bulk of the blood volume. Dielectric loss in the water molecules converts the electric field energy to thermal energy.
The electrodes comprise discrete bands that serially align and space apart from each other. Each electrode has a polarity, and for each electrode there is an adjacent electrode having an opposite polarity. The electric field is generated between the electrodes of opposing polarities, and the electric field extends radially out from the bands to heat flowing blood.
While the electric field is used to heat the blood, it is envisioned that additional forms of heating can be used in conjunction with the heating method of the present invention such as having resistive heating elements in or on the catheter, and having a circulating heating fluid within the catheter.
The system includes a control unit coupled with the catheter via electric cabling for powering the electrodes with radio frequency (RF) energy. The control unit provides alternating current to the electrodes in the RF frequency range (i.e., between 100 kHz to 3,000 kHz). Preferably, the current is at about 500 kHz to generate an electric field of a corresponding frequency.
To optimize the heating effects of the electrodes, the catheter includes a selectively deployable support for positioning the electrodes centrally within the blood vessel. Ideally the support gently holds the catheter within the blood vessel by gently pressing against the walls of the blood vessel. Central positioning of the catheter optimizes heat exchange between the catheter and the blood. The support, according to one aspect of the invention, is adjustable in length.
The support is described in terms of multiple possible embodiments. A according to one embodiment the catheter has multiple supports comprising flattened wires having ends and lengths. The ends of the supports attach to the catheter body. The lengths align longitudinally along the catheter body to selectively deploy against the blood vessel wall to center the catheter body within the blood vessel. Preferably, the distal end of the catheter body includes a switch mechanically coupled with the supports to selectively deploy the supports. Alternately, automatically deployable supports are provided that deploy in response to removal of an insertion tube, or similar device.
Variations of this embodiment include an aspect having the ends of the supports attaching to the catheter body in a position proximal the electrodes. Another aspect has the supports attaching to the catheter body in a position distal the electrodes. Yet another aspect has one end of each support attaching to the catheter body in a position proximal the electrodes, and the other end of each support attaching to the catheter body distal the electrodes.
According to another embodiment, the supports have pins that radially extend from the catheter body to bear gently against the blood vessel.
According to another embodiment, each support includes a ring and extensions. The extensions support the ring to selectively deploy the ring from a first configuration where the ring lies flush along the catheter body to a second configuration where the ring gently presses against the blood vessel. The extensions and ring cooperate to permit blood to flow past the extensions when the ring holds the catheter body centrally within the blood vessel. According to one aspect of this embodiment, the extensions extend at an oblique angle from the catheter body to the ring so that the extensions and the ring form a frustum shape.
One aspect of this embodiment manifests where the ring and extensions form a web. The web circumscribes the catheter body, the web being selectively deployable from a first configuration where the web lies on the catheter body, to a second configuration where the web extends from the catheter body to the blood vessel. The web permits blood to flow through the web when the web holds the catheter body centrally within the blood vessel. The web achieves a frustum shape in the second configuration, according to a variation of the invention. According to a further variation, the web comprises a resistive heating elements which conducts RF current and cooperate with the electrodes to warm the blood.
According to yet another embodiment, the blood vessel has a wall and the support includes a helical wire for contacting the wall of the blood vessel along a helical path. The helical wire is distanced from the distal end of the catheter body to facilitate blood flow between the helical wire and the catheter.